The present disclosure relates to an information processing apparatus for controlling the frequency of a high-frequency signal superposed on a driving signal for driving a laser-beam source and an information processing method provided for the information processing apparatus.
The optical disc such as the DVD (Digital Versatile Disc) and the BD (Blu-ray Disc, a trademark) is widely used. In an operation to reproduce information from a desired position on the optical disc, a laser beam emitted by a laser-beam source is radiated to the desired position and a beam reflected from the position is detected. Then, the reflected beam is converted into an electrical signal in order to generate a reproduced signal also referred to as an RF (Radio Frequency) signal. At that time, a portion of the reflected beam is inadvertently returned to the laser-beam source. The portion of the reflected beam is referred to as a returned beam giving rise to noises of the laser beam. The noises raise a problem in the practical use.
In order to repress the noises caused by the returned beam, a high-frequency signal is superposed on a driving signal which is typically a driving current used for driving the laser-beam source. By superposing a high-frequency signal on the driving signal, it is possible to reduce a RIN (Relative Intensity Noise) which is a noise characteristic of a laser beam generated by the laser-beam source and to increase the S/N ratio (Signal to Noise ratio) of the RF signal reproduced from the optical disc. Such a technique is disclosed in documents such as Japanese Patent Laid-open Nos. 2000-149302 and 2004-288277.
In the optical-disc apparatus, on the other hand, OPC (Optimum Power Control) is executed as processing to adjust the recording power of the laser beam. In accordance with the OPC, in an OPC area provided typically in an innermost circumferential area on the optical disc, tentative data determined in advance is written and the written tentative data is reproduced in order to generate an RF signal. Then, envelope information obtained from the waveform of the RF signal is used to carry out processing determined in advance in order to obtain measurement values which include the modulation degree of the RF signal, a β value and the amplitude of the RF signal. The β value shows the asymmetricity of the amplitude of the RF signal. In the following description, the amplitude of the RF signal is also referred to simply as an RF amplitude. Then, on the basis of these measurement values, a proper recording power of the laser beam is determined. In the following description, the modulation degree, the β value and the RF amplitude are also referred to as a modulation degree which is a generic technical term used to represent the modulation degree, the β value and the RF amplitude. The modulation degree is measured not only during the OPC, but also during a process of adjusting the optical aberration in a signal reproduction operation.